Structural Band Gap Engineering
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ABSTRACT In this paper we discuss a novel approach to the band gap engineering of semiconductors Si, Ge, GaAs and AIN. We suggest that nanoporous polymorphs of these materials may exist which would offer a significant variation of the electronic band gap. Structurally, nanoporous semiconductors are related to the zeolitic nets, and a systematic procedure of generating these structures from the (4;2) nets is described. We use the ab initio total energy quantum molecular dynamics method Fireball96 to investigate the energetics and the electronic properties of these nanoporous or "expanded" semiconductor phases. INTRODUCTION What is band gap engineering? Defined generally, band gap engineering is the alteration of the electronic spectrum (excitation energies, effective masses, etc.) of the material. It is usually achieved either by alloying, by forming superlattices, or by applying strain. In the SiGe alloy system one can continuously change the band gap from that of Si to that of Ge. Superlattices of GaAs and AlGaAs have found a variety of applications. Thin films of Si grown on a SiGe substrate offer a higher carrier mobility due to the tensile strain caused by the lattice mismatch between the substrate and the film. Alternatively, one can try to alter the electronic spectrum of the material without changing the chemical composition or forming superlattices. This can be done by changing the crystal structure of the material; in other words by using different polytypes or polymorphs. Indeed, for SiC going from the cubic 3C polytype to the hexagonal wurtzite 2H (through a series of hexagonal phase such as 4H, 6H etc.) results in a band gap variation from 2.4 eV to 3.3 eV at the same 50/50 composition. This allows one to tailor the optical properties of the material. However, SiC is quite unique with its strong variation of the band gap among the polytypes. Other materials such as ZnS offer fairly small electronic band gap variations (within 0.1 eV) among the polytypes [1]. Polytypism and polymorphism have never been exploited in Si, Ge, or GaAs because it is generally believed that the only stable phase at the normal temperature and pressure is the diamond or zinc blende phase for the elemental and binary semiconductors, respectively. There are of course high pressure phases of these semiconductors, such as 3tin phase of Si, but as a rule these phases are metallic (a metastable BC-8 phase is semiconducting). Recently, it has been proposed, that elemental Si, and C, and binary GaAs may exist in metastable nanoporous structures [2-4]. In contrast to the high pressure (and density) phases, nanoporous semiconductors are "expanded", and have densities which are about 20% lower than those of the corresponding ground state structures. In a pressure experiment one would need to apply a negative pressure which, of course is not easy to achieve. Some of these nanoporous structures known as clathrates have been synthesized in the laboratory, and clathrate phases of Si, Sn, and Ge have been characterized [5].
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